The present invention relates to preparation of a herbicidal composition useful in agriculture and in other situations where control of weeds or other vegetation is desired. In particular, it relates to preparation of a herbicidal active ingredient, namely N-phosphonomethylglycine (glyphosate) in the form of the ammonium salt thereof, as a finished product or as an intermediate useful in further processing.
Glyphosate herbicides, especially herbicides comprising a water-soluble salt of glyphosate, are well known. Specifically, the monoammonium salt of glyphosate is disclosed as a useful herbicide for example in U.S. Pat. No. 4,405,531 to Franz. Unless the context demands otherwise, xe2x80x9cammonium glyphosatexe2x80x9d herein refers to the monoammonium salt of glyphosate, which has the chemical formula 
it being understood that the mole ratio of ammonium cations to glyphosate anions in such a salt is not necessarily exactly 1. A slight molar excess of either ammonium cations or glyphosate anions, for example providing a mole ratio of about 0.8 to about 1.25, is not inconsistent with the term xe2x80x9cammonium glyphosatexe2x80x9d as used herein.
Ammonium glyphosate is the primary salt of choice in the preparation of dry glyphosate herbicide formulations. A xe2x80x9cdryxe2x80x9d formulation herein is a composition that is solid, usually particulate, wherein particles are either aggregated as in a granular composition or non-aggregated as in a powder. The word xe2x80x9cdryxe2x80x9d in this context does not imply that the formulation is necessarily free of water or other liquid, only that it is dry to the touch. Dry formulations can contain up to about 5% by weight of water, but more typically the water content is less than about 1%, for example about 0.5% or lower.
Dry formulations of glyphosate herbicides, like the corresponding liquid (normally aqueous) formulations, typically contain one or more surfactants in addition to the glyphosate salt. Surfactants are important components of glyphosate formulations because, when a glyphosate formulation is diluted, dissolved or dispersed in water for application by spraying to foliage of plants, the surfactants assist in retention of droplets of the spray by the foliage, adhesion of the spray droplets to the foliar surface and penetration of the glyphosate through the hydrophobic cuticle that covers the foliar surface, by these means and possibly in other ways enhancing herbicidal effectiveness of the glyphosate spray.
However, a surfactant is not an essential component of a glyphosate formulation. The end-user can add a non-surfactant-containing glyphosate formulation to a spray tank together with a surfactant, a procedure known as tank-mixing. In some cases, a glyphosate herbicide can be successfully applied without any surfactant. This is particularly true where the dose rate of the glyphosate is rather high, for example above about 1 kg acid equivalent (a.e.)/ha.
Ammonium glyphosate is the preferred salt for use in preparing dry glyphosate formulations for a number of reasons, but perhaps mainly for the reason that ammonium glyphosate is relatively non-hygroscopic. Salts favored for preparation of aqueous formulations, such as the isopropylammonium salt or the trimethylsulfonium salt, are very difficult to dry down to a crystalline state and, once dry, have a strong tendency to reabsorb water. Dry glyphosate formulations based on the ammonium salt are disclosed for example in U.S. Pat. No. 5,656,572 to Kuchikata et al.
The sodium salt, disclosed to be useful in dry glyphosate herbicide formulations for example in International Patent Application No. WO 87/04595, is much less hygroscopic than these salts but nonetheless requires packaging with a very water-impermeable material to avoid absorption of water vapor from the atmosphere and consequent loss of free-flowing properties. U.S. Pat. No. 5,324,708 to Moreno et al. discloses a process for preparing a non-hygroscopic monoammonium glyphosate; however, dry ammonium glyphosate prepared by any known process is adequately non-hygroscopic for most practical purposes.
U.S. Pat. No. 5,266,553 to Champion and Harwell discloses a process for preparing a dry water-soluble composition comprising a salt of a herbicidal compound that includes a carboxylic acid functionality. This process comprises forming an aqueous solution or slurry of such a salt by reacting the herbicidal compound with a neutralizing base in the presence of water, and thereafter removing the water to provide the dry salt. The process is directed particularly at substituted benzoic acid herbicides and phenoxy-substituted carboxylic acid herbicides, but is said to be useful also for glyphosate. The drying method is specified to be one that is controlled such that the temperature of the herbicidal salt does not exceed 80xc2x0 C.; drying under vacuum is preferred, and use of a continuous thin-film dryer or a scraped surface heat exchanger is said to be unsuitable because of the prolonged drying period necessary or because an amorphous paste is produced.
Solid-state reaction of glyphosate acid and ammonium bicarbonate, as disclosed for example in above-referenced U.S. Pat. No. 5,656,572, produces a particulate ammonium glyposate that can be used directly as a herbicidal product, or granulated, for example by pan granulation, to make a dry granular herbicidal product, or further processed with surfactant. Alternatively, an aqueous slurry of glyphosate acid can be reacted with anhydrous ammonia or aqueous ammonia (ammonium hydroxide) to produce a concentrated solution or slurry of ammonium glyphosate. This solution or slurry then has to be dried if the desired finished product is a dry herbicidal composition.
Because anhydrous and aqueous ammonia are much lower-cost sources of the ammonium cation than ammonium bicarbonate, numerous efforts have been made to develop processes wherein glyphosate acid is reacted with anhydrous or aqueous ammonia, yet wherein a dry ammonium glyphosate composition is produced. U.S. Pat. No. 5,614,468 to Kramer et al. discloses such a process wherein solid particulate glyphosate acid is reacted with aqueous ammonia, and U.S. Pat. No. 5,633,397 to Gillespie et al. discloses such a process wherein solid particulate glyphosate acid is reacted with anhydrous ammonia gas.
Processes wherein the acid-base reaction takes place in an aqueous medium, generating a concentrated aqueous solution of ammonium glyphosate, are easier to control than the above solid-state processes. In addition, the exothermic nature of the reaction gives rise to a need for dissipation of heat, which presents much fewer problems in an aqueous medium because of the much greater ease of ensuring adequate mixing and thereby heat exchange than is possible in a solid-state process. There is therefore a long-felt need in the art to develop an efficient aqueous slurry process for making ammonium glyphosate wherein the end-product is a water-soluble dry particulate composition.
Dry particulate compositions of agricultural chemicals such as herbicides are typically powders or granules, for example water-soluble powders or granules. Granules are typically aggregates of smaller primary particles, while powders are typically formed of unaggregated primary particles. Water-soluble powders are not generally well favored because they tend to be dusty. Water-soluble granules are preferred over water-soluble powders, but have a much smaller surface area to volume ratio that tends to retard the process of dissolution, especially in cold water.
The present invention provides a process for making a dry particulate ammonium glyphosate composition that is less dusty than a powder but has a large surface area to volume ratio, promoting a rapid rate of dissolution, even in cold water. The composition takes the form of flakes.